Basic protein fractions from milk have been reported to have various physiological functions, such as bone strengthening effect, preventive effect against periodontal diseases, lipid metabolism improving effect, hypotensive effect, stimulatory effect on dermal collagen production, and regulatory effect on the immune system. For effective exploitation of such physiological functions, various foods, beverages, feed, and pharmaceuticals containing a milk basic protein fraction have been developed. Unfortunately, milk basic protein fractions are heat-labile in the neutral pH region and precipitate when heated at a temperature of 80° C. for ten minutes (see, for example, Patent Document 1). For this reason, milk basic protein fraction are generally heated in the acidic pH region. It is reported that such a method can substantially maintain the bone strengthening effect of the milk basic protein fraction even after the heat treatment (see, for example, Non Patent Document 1).
Unfortunately, such a method still involves problems caused by the low thermal stability of the milk basic protein fraction during sterilization at a temperature 90° C. or more, especially retort sterilization, such as: (1) high tendency for aggregation and precipitation of the milk basic protein fraction during thermal sterilization at a temperature 90° C. or more, due to its inadequate heat resistance; and (2) high tendency for deactivation of the milk basic protein fraction during thermal sterilization, particularly under neutral (with a pH value of approximately 7.0) to alkaline conditions. Accordingly, under actual circumstances, it is impossible to employ an extreme heat treatment, such as retort sterilization, in a case of a heat treatment (particularly at a temperature exceeding 90° C.) for a solution containing a milk basic protein fraction, because such a treatment may deactivate the milk basic protein fraction. Such circumstances have been a limitation in blending a milk basic protein fraction in foods, beverages, feed, or pharmaceuticals without deactivating it.